X-ray imaging apparatus is widely used in medical and veterinary applications in order to obtain images of tissues hidden from view with the naked eye. It is widely understood that whilst x-ray imaging can be of assistance in diagnosis, subjecting a patient to x-rays of itself carries risks. In the case of computed tomography (CT) scanning equipment, the risk posed to patients is very significant. Data published by the UK Health Protection Agency shows that in CT scanning the x-ray energy to which the patient is subjected is so high that for every one thousand people having a CT scan, one person will die as a result of the scan. In some jurisdictions before a patient may receive a CT scan it is a requirement that a consultant give an opinion that the risk to the patient's life from the condition that is being investigated is greater than the risk to the patient's life from having the CT scan.
It would therefore be very desirable in an x-ray imaging apparatus, and in particular in CT scanners, to reduce the x-ray dose received by the patient.
It is known that x-rays having lower energies are more harmful to the patient than higher energy x-rays. This is because the lower energy x-rays are absorbed more readily by the tissues of the body, whereas the higher energy x-rays tend to pass through the tissues of the body with a lower rate of absorption.
It is known that lower energy x-rays may be removed from an x-ray spectrum in a process known as beam hardening. This would reduce the effective dose to the patient. However, the contrast seen by the x-ray detector would be reduced and hence the image quality would be reduced.
The issue of dose reduction has been addressed, particularly in relation to CT scanners where the problem of over exposure to x-rays is most severe.
For example, blocking filters may be used as exemplified in U.S. Pat. No. 8,243,875, in which Xu describes a computer controlled aperture in front of the x-ray source to constrain the x-ray beam angle so that only those areas of interest in an object under examination are illuminated by the x-rays.
Another approach involves active feedback of x-ray attenuation with reduction of dose to minimal needed signal to noise, as exemplified in U.S. Pat. No. 8,406,373, in which Graham describes equipment that would allow variation in X-ray flux using a modulator at different parts of an image to give high signal to noise and corresponding high dose in areas of an image where detail is vital and a lower signal to noise and hence dose in other areas of the image where detail is less important.
Another approach to dose reduction is to combine x-ray imaging with other techniques. For example, in U.S. Pat. No. 8,155,729 Hsieh describes apparatus that gathers x-ray and ultrasound data in cardiac imaging and combines the data. This is said to reduce the number of slices needed in the CT scan, thereby reducing dose.
Another approach involves the mathematical treatment of data. In U.S. Pat. No. 8,363,779 and U.S. Pat. No. 8,199,875 Chandra describes inventions which allow the seamless integration of datasets that are obtained from x-ray beams set at two different energies. The integration of the datasets enhances contrast and gives material specificity. This method has to be timed very carefully so the datasets are collected in such a way that does not increase the dose given to the patient. They are at best dose neutral compared to a standard instrument, but give additional information.
Yet another approach involves improving the component equipment. In U.S. Pat. No. 8,378,310 Bournefalk describes a method to accommodate pulse pile-up in photon counting detectors which improves the signal quality of the detector thus allowing for lower doses to be used for patients when pulse pile-up is present. In U.S. Pat. No. 7,200,201 Unger describes a method of using collimation to reduce the amount of scattered radiation that enters a detector, hence improving the contrast obtained and allowing for lower doses.
Whilst the above described devices may reduce x-ray dose it is understood that the dose reduction levels are marginal. It would be desirable to provide apparatus capable of making significant reductions in x-ray dose without compromising the results of the x-ray imaging.
The main area of use of such an x-ray apparatus would be in human and animal medicine. However, there are other areas of use, in fact such an apparatus would have utility in analysing any body or substance that is sensitive damage by x-rays, such as plant material and polymers for example.